The invention is based on a method for controlling recirculated exhaust gas quantities as generally described hereinafter. In a method of this kind known from U.S. Pat. No. 4,177,777, the quantity of air aspirated by the internal combustion engine is measured by means of an air flow rate meter, and the control pressure of a pneumatically actuatable exhaust recirculation valve is varied in accordance with the air flow rate signal thus controlling the quantity of recirculated exhaust gas. In another embodiment described in this same patent, the quantity of recirculated exhaust gas is measured and compared with a set-point value formed on the basis of the air flow rate signal, and the adjustment of the exhaust gas recirculation valve is then varied in accordance with the resultant correction signal.
In the first method discussed above, the disadvantage is that the recirculated exhaust quantity is merely controlled in open-loop fashion, without being able to detect possible error-producing conditions, which may be associated particularly with the triggering of the exhaust recirculation valve. The control pressure in the pneumatic exhaust recirculation valve of the known type is established with the aid of a magnetic valve, which controls a connection with the ambient air, while at the same time the pressure chamber of the pneumatic valve communicates continuously with the intake manifold downstream of a throttle valve. The vacuum pressure (underpressure) at that location is naturally substantially dependent upon the load and on engine speed, which cannot be compensated for precisely in the first method above.
In the second method discussed above, a measurement apparatus is required for measuring the actual quantity of recirculated exhaust gas supplied, which is expensive. Furthermore, here as well the apparatus for establishing the control pressure as described above, having only a single magnetic valve which controls communication with the ambient air, is disadvantageous, because in every case where the exhaust recirculation valve is not supposed to be completely open, air is supplied to the intake manifold via this magnetic valve but is not detected by the air flow rate measuring means. This is particularly critical in the case where the exhaust gas recirculation valve sticks in a middle position or in the closed position.